1. Field of the Invention
The invention relates generally to solid state relays and more specifically to small solid state relays, under twelve hundred watts, capable of switching AC and DC currents and of being constructed in a single inline package (SIP).
2. Description of the Prior Art
As electromechanical devices, relays have filled an important role in controlling large voltages and currents with relatively low control powers applied to the coils of the relays. Relays have also served to isolate the controlling circuit from the controlled circuit.
Advances in semiconductor technology have allowed the substitution of electromechanical relays with solid state relays. A practical limitation to how much power a solid state relay can switch has been the heat dissipation capability of the actual switching device and its package. Both silicon controlled rectifiers (SCRs) and triacs are used in solid state relays and the load that can be placed on a solid state relay is a function of the SCR/triac device rating and the thermal resistance of device junction to the ambient air (for air cooling).
Schneider describes, in U.S. Pat. No. 4,172,272, issued Oct. 23, 1979, a solid state relay package that has a U-shaped metal frame. The open end of the frame receives a circuit board containing most of the relay components and is potted in place with a solid plastic insulation material. Schneider states that his package doubles the current rating of a given relay circuit because his metal frame is so efficient at removing heat. A triac assembly (31 in FIG. 9) is mounted to the interior bottom of the metal frame by cementing or soldering. The use of a triac in this fashion severely limits the kinds of loads that can be applied to the relay, since an adequate dv/dt change in the control voltage must be maintained. Inductive loads, such as are found with motors, are either curtailed or not allowed at all. The single triac device, as a switch point, tends to concentrate the heat the relay must dissipate, and the efficiency with which heat is transferred from the junction of the triac to the metal frame and then to the air limits the overall power rating of the relay.
An earlier patent by Collins, U.S. Pat. No. 3,723,769, issued Mar. 27, 1973, describes an optically isolated signal circuit for a solid state relay. The relay also has zero-crossing switching with a full-wave bridge to reduce any radio interference that might otherwise be generated. An AC load is controlled by two thyristors connected in inverse parallel such that each handles an opposite half cycle to the other. Collins suggests that a triac can be used to replace the thyristors, and yet does not address the dissipation of the heat generated in any of these devices, while Schneider is addressed to substantially only to the heat dissipation issue.
Copy machines and printers use high intensity halogen lamps that must be turned-on quickly. The initial surge current into one of these lamps can be very high and exceed the capabilities of triac based solid state relays. Halogen lamps have much higher inrush currents than do incandescent lamps. For a given area, a prior art solid state relay having a triac with an eighty amp surge capability will be unreliable because the maximum surge currents are being exceeded. With the present invention, the maximum surge current handling capability can be raised in a device having the same area to 250 amp (using 180 mils .sup.2 chips), and to 500 amp (using 240 mils.sup.2 chips).